This invention is in the field of devices for converting rotary motion to linear motion, and vice versa. Many such devices are well known in the art, some of them centuries old. The treadle is good example of such a device. The beginning of mechanization of labor brought forth many such devices, but the more common ones arose with the invention of the steam engine. Probably the best known and most widely used such device is the crank and lever connection between a reciprocating piston and a rotating shaft. Another well known device is the Scotch yoke.
In the cryogenic compressor art, there is a need for compressors which give a sine wave of pressure in a line. Unfortunately, a conventional reciprocating piston compressor, except for the special case of the instant invention, does not move with a sine wave motion, or provide a sine wave of pressure. In order to achieve such a motion or pressure wave, other means are resorted to, such as the invention shown in U.S. Pat. No. 4,411,164 of Oct. 25, 1983, of which I was a joint inventor. This patent uses a circular cam rotating in an elliptical slot in a yoke to achieve sine wave motion. For the usual reciprocating piston compressor/engine, with a crankshaft, crank, connecting rod, piston, and piston pin, it can be shown that the equation of motion of the piston in the cylinder is as follows: y=a sin .alpha.+.sqroot.b.sup.2 -o.sup.2 cos.sup.2 .alpha., wherein y is the linear position of the piston about some median point, a is crank length (from the center of the crankshaft to the center of the crank pin joining the crack and the connecting rod), b is the connecting rod length (from the center of the crank pin to the center of the piston pin), and .alpha. is the angular position of the crankshaft. It can thus be seen that for unequal b and c, y is not a sine wave, and will depart more and more from such a wave as the ratio of b to c increases. For some cases, this may not be undesirable, but it is in a cryoegenic compressor. My invention, as set forth herein, is a device for converting rotary motion of a shaft to sine wave motion of piston or the like, or vice versa. The device might also be used, for example, to convert ocean wave motion to essentially constant rotational motion.